The production of glass-ceramic articles began with U.S. Pat. No. 2,920,971. The conventional method for preparing glass-ceramic articles involves three general steps: first, a glass forming batch, frequently containing a nucleating agent, is melted; second, that melt is cooled to a temperature at least within the transformation range of the glass and, commonly below that temperature, and a glass article of a desired configuration simultaneously shaped therefrom; and, third, that glass article is heat treated at temperatures above the transformation range of the glass to cause the generation of nuclei in the glass upon which crystals are grown in situ. The heat treatment is typically carried out in a controlled manner so that the crystals developed are quite uniform in size and homogeneously distributed throughout the residual glassy matrix. To accomplish this, the glass article is commonly heated to a temperature slightly above the transformation range to form nuclei therein and then heated to a higher temperature to grow crystals on those nuclei.
In general, the crystals will comprise the greater proportion of the final body and the minor proportion of glass remaining will have a composition far different from that of the precursor glass article, inasmuch as the constituents making up the crystals will have been removed therefrom. Because of the very high percentage of crystals present, the properties exhibited by the glass-ceramic body will more nearly mirror those of the crystal phase than those of the initial glass body.
The size, identity, and character of the crystals developed in glass-ceramic articles are governed through a combination of base glass composition and heat treatment. Hence, crystals of a metastable character can be generated; crystals having compositions and structures not strictly conforming to the classic or standard mineral species. To illustrate, solid solutions, such as .beta.-spodumene solid solution Li.sub.2 O.Al.sub.2 O.sub.3.nSiO.sub.2, are widely found in glass-ceramic bodies and, not infrequently, crystals are developed therein for which there is no authoritative X-ray diffractometry data from the National Bureau of Standards. This situation has led workers in the glass-ceramic field to analogize the crystal structure of an unknown phase to that of a known crystal structure, even though the composition of the unknown phase may be quite remote from that of the analogized crystal.
Whereas in the production of glass-ceramic articles of substantial bulk, a nucleating agent, as such, is normally included in the precursor glass batch to provide the nuclei for the subsequent growth of crystals thereon, that practice is not mandatory. Thus, where desired, a thermally crystallizable glass can be melted and quickly cooled to avoid crystallization. The glass is thereafter comminuted to a finely-divided powder, commonly termed "frit." Upon sintering together into an integral body, surface nucleation occurs which supplies centers for the subsequent growth of crystals as the glass powder is heated to higher temperatures. That practice is particularly useful in preparing coatings and substrate elements of relatively thin cross sections.
U.S. application Ser. No. 258,675, supra, describes the preparation of glass compositions demonstrating low transformation or transition temperatures (Tg), thereby making it possible to conduct melting and forming operations at low temperatures. Hence, those glasses exhibit transition temperatures below 450.degree. C., preferably below 350.degree. C., with working temperatures (temperatures at which the glasses display viscosities of about 10.sup.4 -10.sup.7 poises) below 500.degree. C., preferably between 350.degree.-450.degree. C. As can be appreciated, the maximum long term use temperature for such glasses will be below 450.degree. C., typically below 400.degree. C.
The primary objective of the instant invention was to provide means for increasing the upper use temperature of those glasses, i.e., at least above 400.degree. C. and, preferably, above 450.degree. C., while retaining the forming advantages of low transition temperature without sacrificing the excellent chemical durability exhibited by those glasses.